The present invention relates to implantable stimulation devices, e.g., cochlear prostheses used to electrically stimulate the auditory nerve, or other nerve or muscle stimulators, and more particularly to the electrodes or electrode contacts used with such implantable stimulating devices.
Most cochlear prosthesis currently employ an array of closely spaced electrode contacts implanted in the scala tympani, where they are used to selectively stimulate multiple regions of the tonotopically arranged spiral ganglion cells, which comprise the auditory nerve. In order to minimize the electrical power required to activate the nerve cells, the impedance of the electrical load presented to the stimulating circuitry needs to be minimized. In order to avoid damage to the neurons and other surrounding tissues, the stimulation waveforms must be strictly charge-balanced; i.e., there must be no net direct current flow. In order to simplify surgical implantation and minimize danger of extrusion through the skin, the volume occupied by the electronic circuitry and any associated packaging and connections must be minimized.
In the present art, the above requirements are usually met by employing metal electrode contacts, and connecting such electrode contacts to hermetically sealed electronic circuits via leads and feedthroughs into the hermetically sealed electronic package. The electrode contacts typically consist of the noble metal platinum and its alloys with iridium. This choice gives rise to the following design problems and strategies for coping with them: